Most electronic circuits on rigid substrates are still made by well known methods of etching copper-clad boards. A rigid dielectric substrate is first covered with copper or other electrically conductive material. Then, after temporarily protecting that portion of the conductive layer that will form the desired circuit trace, a substantial amount of this layer is chemically etched away. The protection is then removed, leaving copper traces in desired locations. One or more circuit components are attached to the circuit traces, usually by melting eutectic tin-lead solder.
Highly toxic materials used in these conventional methods have been, and continue to be, of great concern to individuals and the environment. Large amounts of water used in such methods have been discharged from manufacturing sites bearing trace amounts of metallic residues (lead, tin and copper, for example) as well as cyanides and other harmful wastes. Millions of rejected, worn out or obsolete electronic products containing such substances have been disposed in landfills where they are, or some day may be, pollutants of runoff or ground water. Additionally, millions of tons of ozone-destroying chlorofluorocarbons ("CFCs"), used to clean solder residues, have been released into the atmosphere from this source alone.
Planar subsurface electronic circuits made by filling channels in rigid dielectric substrates with electrically conductive materials, though not new per se, are not extensively used. For example, IBM Technical Disclosure Bulletin Vol. 27, No. 5, October, 1984, "Subsurface Circuitry On Polymeric Substrates", by E. W. Mace, discloses a method of forming subsurface circuits in an injection-moulded substrate having angled-wall channels. A compliant elastomeric blade or squeegee was used to fill the channels with a polymeric thick film conductive paste. The paste was drawn across the surface of the substrate using the squeegee to fill molded-in circuit lands. The substrate was then turned 90.degree. and the filling process was repeated. The paste was dried and cured to its final state, following which excess cured paste was buffed away. Only then were circuit components attached to the cured conductive-paste-filed channels by various methods, including soldering.
This method has several characteristics. First, it takes two passes to fill the dielectric with the conductive paste. Second, unnecessary waste is produced because the substrate's surface must be sanded or buffed after the channels have been filled and cured to remove excess paste. Yet a third characteristic is that circuit components are placed onto the filled channels after the paste is cured. Thus, an additional step of electrically connecting the components to the paste is needed. Typically, this is achieved by soldering, which contains lead and is harmful to the environment.
U.S. Pat. No. 4,645,733 to Sullivan teaches a process which involves steps of: 1) covering a printed wiring substrate with a layer of photopolymer paste, 2) photographically producing a wiring pattern in the photopolymer, paste layer, 3) washing out uncured photopolymer to leave indentations in the cured photopolymer 4) using a squeegee to fill the indentations with a conductive material, such as a polymer ink, and 5) hardening the conductive material. However, residual conductive material remains on the surface, and must be removed by sanding.